The present invention relates to high energy laser beam attenuation; and more particularly, to an improved method and system for producing an attenuated fiducial replica of a high energy laser beam.
High energy laser beams are typically defined as having a peak power in the neighborhood of 15 megawatts; and may be collimated in the form of a circular beam or an annular beam having a large external diameter, in the neighborhood of 8 to 12 inches, for example, in order to achieve small beam divergence. With the advent of increased use, it has become increasingly necessary to characterize high energy laser beams, in terms of beam divergence, energy distribution in the far field footprint, and centroid behavior of beams. In order to accomplish such characterization, it is necessary, because of such high power, to produce a highly attenuated fiducial replica of the far field pattern of the beam to be characterized, having total power in the neighborhood of a fraction of a watt, for example, so that desired measurements may be derived from appropriate sensors placed in the focal plane of the beam.
The current practice of producing a high energy laser beam replica involves intercepting the laser beam by a two dimensional diffractive beam sampler, which exhibits a very small power transmittance in the diffracted orders; and diverting the bulk of the laser energy into an optical absorber, or dump, in an essentially non-dissipative manner. The low energy transmitted diffracted orders are collected by suitable long focal length focusing optics to produce an attenuated far field fiducial replica of the incident high-energy laser beams in the focal plane. Heretofore, a long focal length (e.g., 40 meters) was necessary in order to obtain a replica of sufficient size for proper resolution when characterizing beams of small divergence.
A two-dimensional beam sampler that is widely used for attenuation is a diffraction grating that is comprised of a thin highly reflective plate that is perforated with an array of small holes to provide a representative transmittance sampling over the entire area of the collimated beam. Such a two dimensional beam sampler has the ability to accommodate very high incident peak and average power levels; and the zero order, or on-axis, beam is relatively insensitive to tolerance levels in hole placement, or shift in hole locations, caused by thermal drift. The diameter and density of these holes control the fraction of incident laser power that is transmitted through the grating. The goal is to transmit the smallest fraction of laser energy, while obtaining a true representative sampling of the entire beam; and yet provide adequate angular separation between the zero order beam and adjacent diffracted orders to fully resolve the zero order beam and its sidelobes. This is achieved by the proper selection of hole spacing, which, of course, in turn dictates hole density, i.e., number of holes per unit area. Once hole density is fixed, the grating transmittance is determined by the selection of hole diameter.
Prior to the present invention, a very small grating transmittance, and thus very small holes, were required in order to non-dissipatively deflect the bulk of the laser power. For example, if the peak zero order power of an attenuated fiducial replica in the focal plane is to be less than 0.5 watts, and the peak input power to the diffraction grating is 15 megawatts, a grating transmittance of 1.79.times.10.sup.-4 is necessary. Thus, if an 800 micro-radian separation of adjacent orders is necessary to adequately resolve a number of 100 micro-radian features, for example, the spacing density of the holes require hole diameters of approximately 0.008 inches.
Since it is essential that true hole diffraction occur, rather than diffraction from a deep hole which simulates a cylindrical wave guide, the hole depth should not exceed approximately 25 percent of the hole diameter, or 0.002 inches, in the present example. This, of course, requires that the diffraction grating be made of (1) a very thin plate, which, of course, is undesirable because of lack of rigidity and lack of capability to preserve the flatness of the reflective surface; or (2) a thicker plate, with the small holes being counterbored to within a few mils of the opposite surface, which further increases the difficulty of fabrication. Further, because of the mounting of the diffraction grating at an angle (typically 45.degree.) with respect to the optical axis of an incident laser beam for convenient deflection requiring that the axis of the very small shallow depth holes be at an angle to the plane of the reflective surface of the plate, such a grating fabrication is further complicated.
As previously mentioned, to produce a replica which has a large enough image size in the focal plane, to match the image size of an available sensor area, for maximizing image resolution, very long focal length optics are required. For example, in order to match a sensor area that is 17 millimeters in diameter, which is required for a 400 micro-radian field of view, a focal length of approximately 42 meters is required. Since there is a particular need for field deployable replica producing systems, which may be readily transported on an over-the-road vehicle, the actual physical length of the system, as well as its overall size, should be minimized. Compact test optics are desirable in normal environments but particularly so where mobility is at stake. Specifically, it is desirable that any linear dimension of a system for producing an attenuated replica be in the neighborhood of one and one-half meters, for example. The use of a Cassegranian optical system may be utilized to shorten the physical length of the optics while still maintaining a long effective focal length, but such a resulting system would still be longer than would be convenient for a vehicle transportable system. Further, the use of relay optics may also be considered to increase the effective focal length of the system, while still saving space, but the magnification must be kept low in order to preserve image quality when using existing camera optics, such as a Pyro-electric vidicon camera, for example, as a sensor.